1. Field:
The instant invention relates to leak detectors for detecting very small leaks in small volume enclosures.
2. State of the Art
Existing leak detectors for detecting very small leaks existing in small volume enclosures generally involve a mass spectrometer. An enclosure to be tested, for example, a tube with one closed end and one open end, is sealed to an open-ended conduit of the leak detector. A vacuum is applied through the conduit to the tube to evacuate the system. The exterior of the tube is then sprayed with helium gas so that any fissures in the tube will permit helium to pass from the outside of the tube at a pressure close to one atmosphere into the vacuum very low pressure environment inside the tube. The helium is then transported through the leak detector device to a mass spectrometer which measures the amount of helium present.
The mass spectrometer leak detectors are accurate when used carefully. However, a mass spectrometer is a relatively complicated device and it is necessary to have a rare gas such as helium present to eliminate artifacts.
A typical mass spectrometer device is illustrated in FIG. 1. This device employs a tungsten element which frequently burns out if the device is operated without a device to be tested in place. Any significant quantity of oxygen reaching the hot tungsten filament causes it to oxidize rapidly.
Also, the sample to be tested is utilized by "spraying" helium on the part to be tested. Since only helium ions are detected within the mass spectrometer, unless the sample is completely bathed in helium to the exclusion of all air, an extremely accurate quantitative determination of a leak will not be possible.
Sensors of various types utilized ion-conducting membranes. Exemplary of sensor devices employing zirconia membranes are those disclosed in U.S. Pat. Nos. 3,843,400 of Radford, et al., and 3,650,934 of Hickam, et al. Most sensors of this type operate at ambient pressures and measure electromotive force generated by oxygen partial pressure differentials as being indicative of oxygen concentration on one side of the membrane when a reference gas of known oxygen concentration is present on the other side of the membrane. Sensors of hydrogen ions may include electrolytes of a ceria ceramic material containing strontium oxide and yttria.